Time interval measubing device



June 20, 1950 E.. G. Newsom TIME INTEEVAL MEAsuEING DEVICE Filed July 24, 1946 JNVENTOE 4,62 62 NEM/50417 as frag/vzw- Patented June'ZO, '1950 l amended April 30, 1928; 370 O. G. 757) The invention described herein may be manufactered and used by `or for the Government for governmental purposes without payment to 'me of any royalty thereon.

The present invention relates to radio distance-measuring apparatus and more particularly to an improved circuit for measuring the duration of short time intervals in such `systems.

In an earlier application entitled Radio System," Serial No. 641,161 filed January 14, 1946, of which the present application is a continuation in part, I described radio distance-measuring apparatus suitable for use in controlling airport trailic. Reference was made therein to a trigger circuit, which formed an important feature of my earlier invention. Basically, the aforementioned trigger circuit teaches how extremely short time intervals may be measured by employing two thyratrons, whereby the :first thyratron is utilized to start charging a capacitor and the second thyratron is utilized to interrupt the charging of the capacitor, and the magnitude of charge on the capacitor is determined by a suitable indicator, such as a. voltmeter. This indicator when calibrated in terms of time, registers the duration of the charging period.

VMy present invention teaches how a direct measurement of time with a high degree of accuracy can be obtained by approximately matching an exponential voltage by varying a linear resistance.

The primary object of this invention is to provide means for measuring short time intervals by varying the voltage across a linear resistance, suitably calibrated in terms of time, until a balance point is indicated on a meter.

It is a further object of this invention to provide novel means of translating the maximum level of rise of an exponential voltage on a capacitor into terms of elapsed time during rise without having to resort to the use of a nonlinear, specially wound potentiometer which is highly expensive.

Other objects, features, and advantages of the present invention will become apparent to those skilled in the art when the following description is read'in connection with the accompanying drawings in which:

Fig. 1 is simplified schematic diagram illustrating the principle of my invention;

Fig. 2-is a schematic illustration of my invention as embodied in radio distance-measuring apparatus; and

Fig. 3 is a comparison of typical exponential and hyperbolic characteristic curves Within a limited range, with their differences exaggerated. Referring to Fig. l, a voltage divider 9 isconnected across power supply I0 which provides, for example, 500 volts to ground or negative terminal of power supply IU, as shown. For the time being, a switching device II is shown in Fig. 1 merely to illustrate a particular principle, and in Fig. 2 it will be shown how the functions performed by switch II, Fig. 1, may be performed automatically.

Switch II comprises a movable switch arm I2 connectable to` any one of three stationary terminals. A memory capacitor I3 is connected between switch arm I2 and ground or negative terminal of power supply I0, as shown. A movable tap I4 on voltage divider 9 is adjusted to a point having approximately three fths of the voltage of the divider. An initially charging capacitor I5 is connected between movable tap I4 and ground or negative terminal of power supply I0, as shown to provide a low impedance source of potential. Between tap I4 and a first stationary terminal I6, is connected a variable resistor I1 that also is in series with capacitor I5 between terminal I6 and ground, or the negative terminal of power supply I0. A second stationary terminal I8 is connected directly to ground or negative terminal of power supply I0, as shown.

It will be seen that if switch arm I2 is connected to terminal I6, capacitor I3 will assume a charge from across the plates of capacitor I5 and will charge exponentiallyvat a rate depending on the value of resistor Il, to a level of voltage approaching 300 volts, dependingr on the period of time during which switch arm I2 is connected to terminal I6. Switch arm I2 may also be connected to terminal I8 to discharge capacitor I3 to ground, in order that a succeeding voltage placed on capacitor I3 may `build up from a zero value.

According to the potentiometer .method of measuring voltage, the voltage across capacitor I3 may be measured by matching it to a known voltage of corresponding polarity. The matching voltage may be obtained from a rheostat suitably supplied from voltage divider 9. To accomplish the aforementioned measurement, I have connected a resistor I9 in series with a rheostat 20, across voltage divider 9, resistor I9 being connected to the high voltage end of divider 9. Between the junction point of resistor I9 with rheostat 20, and the third stationary terminan 2|, Ar have eating meter 22. auch as an electronic voltmeter 2,51 1,ses

3 connected a Asuitable indihaving a high input resistance for example. The movable arm 23 of rheostat 2l may be so adjusted that the voltage across the rheostat may be made equal to the voltage on capacitor I3 whenswitcharmisconnectedtoterminal 2|. tion exists across capacitor |3 and rheostat 2l. may be determined byobserving meter 22.- During the time the charge on the capacitor I3 is being read, the capacitor I l is arriving asaln at a new balanced charge that is not applied to the capacitor Il untilthe switch arm I2 yhad been successively applied to terminals Il and I6.

let us assume that switch arm I2 is connected to terminal Il until capacitor I3 is completely discharged. If switch arm I2 is then moved to terminal II for a given period of time "t, during which capacitor I3 buildsup a voltage of, let us say 150 volts, after which switch arm |2 is connected to terminal 2|, then rheostat arm 23 may be moved in a direction indicated by meter 22 until the voltage across rheostat 2l corresponds to the voltage across capacitor I3. To insure accuracy in making the nnal adjustment, it may be necessary to repeat'the aforementionedv cycle so that the 150 volts initially gained bythe capacitor I3 will not be allowed to decay while the rheostat voltage is being brought up to the value of the capacitor voltage.

In actual practice, the switch function performed by switching device il is accomplished by electronic and mechanical means to demarcate the charging period more sharply and to increase the repetition rate of the dischargecharge-measurement" cycle.

Let us further assume theiollowing definitions of symbols:

E9=Voltage across divider 3,

E1=Voltage across capacitor I3,

E14=Voltage at tap I4,

E22=Voltage across meter 22,

R1v=The resistance of resistor I1,

R19=Resistance of fixed resistor I9,

Em :Voltage at the contact point oi' rheostat arm t13=Charging time of capacitor I3,

C1a=Capacitance of capacitor I3,

Rm=Resistance at the contact point of rheostat arm 23,

K=A constant.

.maticallyhw-'the principle employed in Fig. 1

may be applied -to a preferred embodiment of my invention 'in a radio distance-measuring trigger circuit similar to that disclosed in Fig. 4 of my aforementioned copending application.

A pulse initiating the ltime interval to be measured is introduced at terminal with the switch 35 closed and the switch 36 open. The terminal 3|! is connected to the control grid of a thyratron tube 3|. The arrival of the pulse causes the tube 3| to conduct and charge capacitor 42. The $011- Whether or not a balanced voltage condi- .4 trol grid oi' tube '3| is connected to the cathode thereof through grid resistor 32 and a suitable bias voltage source 33 which normally keeps the tube in a noncondueting state. A suitable motor 34 actuates the switches 35 and 36 in a sequence which will become evident as the description progresses. 'I'he switch- 35 remains closed and the switch 36 open for a time slightly longer than the time interval between the initiating pulse and a returned pulse or echo. A second thyratron 31 is provided, its anode being connected to the anode oi thyratron 3|, the anodes of both thyratrons being connected through switch and variable resistor Il to a tap 41 on a suitable source oi' positive voltage such as a voltage divider 43 connected across power supply 39. 'I'he control grid of thyratron 31 is connected to its cathode through a grid resistor 4|I and a suitable bias voltage source 4I, which normally keeps the thyratron in a nonconducting state. The cathode oi' thyratron 31 is connected to ground, while the cathode of thyratron 3| is connected through a capacitor 42 to a tap 49 at some point above ground potential on divider 48, say for example at 28 volts. A capacitor III is connected between tap 41 and tap 49. Another capacitor 5I is connected between tap 49 and ground. These capacitors are provided for the purpose of having a low impedance source of potential. The returned pulse terminating the time interval is applied to terminal 43 in the trigger circuit shown in Fig. 2, causing tube 31 to conduct, stopping the conduction of tube 3| and stopping the charging of the capacitor 42. The magnitude of charge on the capacitor 42 depends upon the length of the time interval between the initiating and terminating pulses.

At a, point intime after the firing of thyratron 31 but before the next initiating pulse, the motor 34 opens switch 35 and moves switch 36 from the open position to position A. 'Ihe plate voltage supply to both thyratrons is thus interrupted, and thyratron 31 returns to a nonconducting state, thyratron 3| already being in a nonconducting state as explained previously. With switch 36 in position A, a memory capacitor 44 is charged through a resistor 45 by the charge developed across capacitor 42 during the measured time interval. Capacitor 44 is preferably of a relatively small value to minimize the slight error introduced in the process of charging it. Switch 36 is then moved to position B and remains there long enough to allow capacitor 42 to discharge through resistor 46. Switch 35 is then closed by the motor 34 and the switch 36 is returned to the open position before the next initiating pulse is applied to the control grid of thyratron 3|, whereupon the cycle is repeated.

A low pulse repetition rate may be employed, a complete cycle occurring, for example, once every second, the memory capacitor 44 main.

taining the indicating charge.

It will now be shown how the method of measurement taught in Fig. 1 is adapted to the trigger circuit of Fig. 2 to determine the level of charge on capacitor 44. A resistor 52 in series with rheostat 53 is connected across voltage divider 48, resistor 52 being connected to the high voltage end of divider 48. Between the junction point of resistor 52 with rheostat 53, and resistor 45, is connected a suitable indicating meter 54, such as an electronic voltmeter having a high input resistance. A movable arm 55 of rheostat 53 may be so adjusted that the voltage across the rheostat may be made equal to the voltage on 'atrasos capacitor 4d. A calibrated scale 58 on rheostat 53 will thus provide the observer with continuous and accurate range infomation in terms of time or directly in terms of distance, as desired.

Fig. 3 illustrates the similarity between two curves, 25 and 28, within an initial range. Curve 25 is typical of the exponential rise of voltage on capacitor I3 in the circuit shown in Fig. 1. Curve 2B illustrates the hyperbolic rise of voltage across rheostat 20, Fig. 1, versus linear variation of the rheostat arm 23. It will be noted that within the time of the time constant RrzCn of the exponential curve, that both curves 25 and 26 nearly coincide. linearly'wound rheostat, such as rheostat 20 of Fig. 1, may be utilized in combination with a resistor, such as resistor I9, in lieu oi' an expensive non-linear potentiometer. Beyond the time of the time constant of curve 25, the error increases substantially. However, the preferred embodiment of my invention is designed to measure periods of time which are no greater than that of the time constant of the circuit employed.

It will be observed that identical periods of charge of capacitor 42, Fig. 2, will result in identical levels of capacitor voltage, provided of course that prior to each charge, capacitor .s2 has zero voltage thereon. Therefore, the period of charge, as defined by the interval between demarcating pulses applied to terminals 30 and 43 respectively of Fig. 2, will always be a function of time, and if the demarcating pulses represent transmitted and reflected radar pulses respectively, then the period of time may be translated into terms of distance.

Small fixed errors in the system can be corrected by proper calibration of meter 54 and by proper adjustment of resistor 38.

The trigger circuit herein described is capable of measuring extremely short time intervals in the order of small fractions of a microsecond. By employing two thyratrons with their cathodes at different fixed potentials, the time intervals may be demarcated by the ionization times of the thyratrons, one initiating the time interval, and the other terminating it. Furthermore, the present invention permits the use of very low pulse repetition rates thereby simplifying a radio control system for aircraft traiiic where many channels may be required.

It should be understood that the time meas- I uring system as set forth herein may be advantageously employed wherever time intervals be'- tween discrete pulses are to be measured with accuracy. Thus, the present invention may find use in pulse-echo radar systems where a slow repetition rate and high accuracy are important factors.

I claim:

1. A radio circuit for measuring short-time intervals, comprising a power suppy, a voltage divider connected across said power supply, a load circuit connected across said voltage divider, a fixed resistor in said load circuit, a rheostat in series with said xed resistor in said load circuit and having a scale, an adjustable arm for varying the resistance introduced by said rheostat in said load circuit, a switch having a movable switch arm adapted for sweeping across a plurality of switch contacts, a first capacitor connected between said switch arm and ground, a ground contact engageable by said switch arm for grounding both plates of said capacitor and eiecting substantially the complete discharge thereof, a second contact engageable by said Therefore, Within this initial range, a

switch arm, a variable resistor connected to said second contact. a tap adapted for being adjustably positioned along said voltage divider and con-v indicating a potential balance between said nrstcapacitor and said rheostat upon the adjustment of said rheostat arm upon said rheostat so that the rheostat scale reading is a meaurement of the time interval during which said first capacitor is charged.

2. A radar range indicator for measuring the duration of short-time intervals, comprising a power supply, a voltage dlviderconnected across said power supply and providing direct current potential to a load circuit, a fixed resistor in the load circuit, a variable rheostat in series with said fixed resistor in said load circuit, first capacitor means connected across a predetermined portion of said voltage divider and chargeable thereby during a time interval between the reception of a pair of initiating and terminating pulses, a switch means having a movable arm connected to one slide of saidV iirst capacitor, a contact engageable by said switch arm. a meter connected between tie junction of said iixed resistor with said rheostat and said switch contact, a second capacitor connected between said switch contact andthe low side of said voltage divider whereby said first capacitor is adapted to charge said second capacitor upon causing said switch arm to engage said switch contact whereupon said adjustable rheo-` stat may be brought to substantially the same potential as said second capacitor as indicated upon said meter for providing a rheostat reading comparable with the time interval between initiating and terminating pulses.

3. A time measuring radio circuit, comprising a direct current power supply, a time measuring capacitor adapted for receiving a charge from said direct current power supply, a switch com'- prising a switch arm connected to said capacitor and sweeping a plurality of contacts, its engagement with one of said contacts being adapted for discharging said time measuring capacitor preparatory to receiving a charge, a variable resistor connected by operation of said switch arm to be placed in series with said capacitor for adjusting the magnitude of the charge imposed thereupon from said power supply, a fixed resistor associated with said power supply, a rheostat for being balanced against the charge upon said capacitor by operation of a movable arm and connected in series with said iixed resistor across said power supply, and a null current indicating means` connected between a contact of said switch and the junction of said iixed resistor with said rheostat for indicating the absence of current flow between said time measuring capacitor and said rheostat upon the adjustment of the rheostat arm whereby a time interval between successive positioning of the arm of said switch with said variable resistor and with said null current indicating means may be measured.

4. A distance measuring accessory to radio apparatus, comprising a direct current source, a voltage divider across said direct current source, a high voltage first tap engaging said voltage divider, a rst electronic tube having a plate,

control grid, and cathode and adapted for receiving an initiating pulse upon its control grid for tap engaging said voltage divider at a position of lower potential than the position of said first tap for applying a predetermined voltage determined by the settings of said taps across said first tube, a large capacity storage first capacitor connected between the cathode of said first tube and said second tap and assuming a charge from across the portion of said voltage divider between said taps upon the conduction of said first tube, a second tube having a plate connected to the plate of said first tube and having a control grid and a cathode and adapted for' terminating the conduction of said first tube upon the application of a terminating pulse to the control grid of said second tube and terminating the period of charge for said first capacitor, a first switch means releasably conducting plate potential to both tubes during the charging of said storage first capacitor and then opened momentarily to interrupt the tube plate voltage supply, a fixed resistor connected to thehigh voltage terminal of said voltage divider, a rheostat in series with said fixed resistor across said voltage divider and having a movable arm connected with the low voltage side of said voltage divider, a small capacity memory second capacitor having one terminal connected to the low voltage side of said voltage divider, a null voltage indicating meter having one terminal connected to said memory capacitor and another terminal connected with the Junction of said fixed resistor and said rheostat, a second switch means having a movable arm connected through said first capacitor to said low voltage second tap .on said voltage divider and to the cathode of said first tube and adapted for selectively engaging a grounded switch contact connected to the low voltage side of`said voltage divider for discharging said first capacitor preparatory to applying a charge thereto and for selectively engaging a memory capacitor switch contact connected between said meter and said memory second capacitor for transferring charge from said storage iirst capacitor to said memory second capacitor preparatory to causing said second switch means to assume an open position in which position said storage first capacitor may be recharged'during the adjustment of said rheostat arm in balancing the resistance of said rheostat against the charge on said memory second capacitor to a null position upon said meter provides a distance measurement from said rheostat.

5. The combination of a power supply providing a direct current voltage to a distance measuring circuit, a voltage divider connected across said power supply,- a load circuit connected.

across said voltage divider, a first thyratron having a plate adjustably connected to a high voltage contact on said voltage divider, a first thyratron control grid to which a time interval initiating pulse may be applied for causing said thyratron to conduct, and a first thyratron cathode, a first capacitor connected between the cathode of said first thyratron and a -low contact on said voltage divider, a second thyratron having a plate connected to both the plate of said rst thyratron and releasably to the high voltage contact on said voltage divider. a second thyratron control grid to which a time interval terminating pulse may be applied for causing said second thyratron to conduct and terminate the conduction of said first thyratron, and a second thyratron cathode connected to the low side of lcausing said tube to conduct, a low voltage second said load circuit on said power supply. a first switch means for terminating the conduction o! said second thyratron, a second switch means comprising a plurality of contacts and a switch arm connected to said first capacitor in series with the low contact on said voltage divider, a second capacitor connectable through said second switch means to said first capacitor to arrive at a potential in balance with the potential of said first capacitor during the time interval between initiating and terminating pulses and to disconnect said capacitors from each other, a fixed resistor in series on the load circuit side of said voltage divider, a rheostat comprising a rheostat resistor in series with said fixed resistor on the load circuit side of said voltage divider through a rheostat arm adjustably connecting the low side of said load circuit with said rheostat resistor, a rheostat scale across which said rheostat arm is adapted -to sweep, and a meter in series with said fixed resistor and said second capacitor on the load circuit side of said voltage divider, said meter and second capacitor being connected in series across said rheostat so that a relative potential acrossV said second capacitor may be balanced by the adjustment of said rheostat arm upon said rheostat resistance to provide a reading of the distance traveled by signal between the initiating pulse and the terminating pulse while said first thyratron is applyinga new charge on said first capacitor.

6. A circuit for measuring a time interval between an initiating pulse and a terminating pulse, comprising a direct current power supply, a voltage divider having a high side and a l'ow side and connected across said power supply, a fixed first resistor having one terminal connected to the high voltage side of said voltage divider, a rheostat having a movable arm connected with the low voltage side of said voltage divider and a rheostat resistance engaged thereby to be ad- Justably connected in series with said fixed first resistor across said voltage divider, a meter having a first terminal connected with the junction of said fixed resistor and the resistance of said rheostat, a memory first capacitor connected between a second terminal of said meter and the low voltage side of said voltage divider, a first higher potential tap adjustably contacting said voltage divider, a second lower potential tap adjustably engaging said voltage divider to provide a predetermined potential between said first tap and said second tap, a second capacitor between said first tap and said second tap, a third capacitor between said second tap and the low side of said voltage divider, a first thyratron having a plate, a control grid, a cathode and a screen grid, a second thyratron having a plate, a control grid, a screen grid and a cathode and having its plate connected with the plate of said first thyratron, a first switch adapted for connecting said first tap with'the plates of both of said thyratrons. a variable se'cond resistor connected between said first switch and said first tap, a time charged fourth capacitor connected in series between said second tap and the cathode of said first thyratron, a control grid bias applied to the control grid of the first thyratron, an initiating pulse terminal connected with the control grid of said first thyratron. for the control grid of said second thyratron, a terminating pulse terminal connected with the control grid oi' said second thyratron, the screen grid and cathode of said second thyratron being connected to the low voltage side ci a control grid bias A said voltage divider, a movable arm connected through said time charged fourth capacitor to the lower potential second tap on said voltage divider and said movable arm connected to the screen grid and the cathode of the said iirst thyratron and adapted for engaging either of two contracts, the arm of the second switch engaging one of its contacts for causing the discharge of said fourth capacitor preparatory to the application of a charge thereto by the conduction of said iirst thyratron upon the application to the control grid thereof of an initiating pulse, and the arm of the second switch engaging the second of its contacts for transferring the charge from said fourth capacitor to said memory first capacitor, a resistor connected between the rst contact of said second switch and the low voltage side of said voltage divider, a resistor connected between said second contact of said second switch and the junction of said meter with said memory rst capacitor, whereby said second switch is opened preparatory to the adjustment of the movable arm on said rheostat to a position at which said meter indicates no current flow between said memory iirst capacitor and said rheostat at which the reading of said rheostat arm indicates the duration of time between the initiating and the terminating pulses.

7. A radio distance measuring apparatus, comprising in combination a voltage divider provided with tap means for drawing a desired potential therefrom, a first thyratron across which the fractional potential from the voltage divider may be releasably applied and having positive and negative electrodes and a control grid, an initially charging rst capacitor connected between the negative electrode of said first thyratron and the lower potential of the fractional potential drawn from said voltage divider and assuming a charge upon a signal being impressed against a negative bias upon the control grid of said rst thyratron and causing said thyratron to re, a second thy rati-on having a positive electrode connected with the positive electrode of said thyratron and releasably connected with the upper tap of the fractional potential drawn from said voltage divider and having a negative electrode connected with the lower terminal of said voltage divider and having a control grid to which a second signal may be applied against a negative bias for causing the second thyratron to re and extinguish the rst thyratron and terminate the applica.- tion of charge to said initially charging capacitor, a memory second capacitor adapted for being removably connected across said initially charged first capacitor and a portion of said voltage divider, resistor means shunting said voltage divider and consisting of a xed resistor connected in series with a variable resistor, a meter connected between the junction of the xed and variable resistors and a plate of said memory second capacitor adapted for being releasably connected to the cathode of said rst thyratron, and a distance indicating arm on said variable resistor movable therealong in arriving at a match with the electric charge on said memory second capacitor indicated by a null reading on said meter with the connection between the memory second capacitor and the cathode of said rst thyratron interrupted.

8. In a time interval measuring device, a iirst circuit responsive to change in condition at the beginning and end of the interval to be measured; a large capacity storage first condenser in said rst circuit, a, rst electronic tube in said nrst circuit and having a cathode circuit in which said storage nrst condenser is connected and said rst tube being placed into conduction at the start of the period to be measured for charging said storage mst condenser during such period of conduction, a second electronic tube in said first circuit and adapted for terminating a period of charge on said storage first condenser by quenching the conduction of said rst electronic tube; a time interval indicating second circuit, a memory second condenser in said second circuit, a null indicating meter in said second circuit, an adjustable variable potentiometer in said second circuit and across the terminals of which said second condenser and said meter are connected in series, switch means releasably connecting said rst and said second circuits for imparting a fraction of the charge on said storage iirst condenser upon said memory second condenser whereupon the variable potentiometer may be adjusted to indicate the length of the time interval to be measured by being caused to oppose the charge on said memory second condenser as indicated by said null indicating meter, and means for discharging said storage first condenser preparatory to the application of a new charge thereto.

9. In a time interval measuring device, a rst circuit responsive to a change in condition at the beginning and end of the interval to be measured; a irst electronic tube having a cathode and connected in said iirst circuit, a large capacity storage first condenser in the cathode circuit of said rst electronic tube and charged during the period of conduction of said rst tube, a second electronic tube in said rst circuit for quenching the conduction of said rst electronic tube and the charging of the said storage i'lrst coridenser at the end of the time interval to be measured, a time measuring memory condenser containing second circuit connected with said first circuit, a variable potentiometer graduated in time intervals in said second circuit, a memory second condenser in said second circuit, a null indicating instrument connected in series with said second condenser across said variable potentiometer, switch means releasably connecting together then disconnecting said first and second condensers whereafter the charge on said memory second condenser assumed from said rst condenser is balanced by the setting of the potentiometer at the null indication on said instrument as a measure of the time interval during which the said first electronic tube starts and ends its period of conduction.

10. In a time interval measuring device, a iirst circuit responsive to change in condition at the beginning and end of the interval to be measured; a low impedance source of potential supplying a predetermined voltage to said rst circuit, a large capacity storage rst condenser in said first circuit, a normally nonconducting rst electronic tube having a cathode circuit in which said rst condenser is connected and is charged during the period of conduction of the first tube and said first tube having a plate and a control grid normally biased to cutoff; a normally nonconducting second electronic tube means having a plate connected with the plate of said first tube and quenching the conduction of said first tube and arresting the charging said storage vlrst condenser at the end of the time interval to be measured; a time interval indicating second circuit, a small capacity memory second condenser in said second circuit, switch means releasably connecting said nrst and second condensers in ll parallel to ground and imparting charge from said iirst condenser `to said second condenser, a variable potentiometer in said second circuit and having a scale calibrated to read in units of time, a null indicating instrumentconnected in series with said memory second condenser across theJ terminals of said potentioleter so that the reading on the potentiometer scale with the indicating instrument at Ia null position indicates the time duration of conduction of said ilrstr electronic tube.

1l. In a time interval measuring device, a rst circuit responsive to change in condition at the beginning and end of the interval to be measured; a time interval indicating second circuit, a power supply applied directly to said second circuit, voltage divider means shunting said power supply and variably tapped intermediate its ends for supplying potential to said first circuit, a rst thyratron in said first circuit and having plate,

vgrid and cathode electrodes and normally biased for nonconduction, a large capacity storage first condenser connected between the cathode of said rst thyratron and said voltage divider s that the first condenser assumes charge during the conduction of said first thyratron following the application of signal to the grid thereof, a normally nonconducting second thyratron in said ilrst circuit and having plate, grid and cathode electrodes with its plate electrode connectedto the plate of said first thyratron and its cathode connected to the low side of said power supply, switch means connecting the cathode of said rst thyratron to said second circuit, variable potentiometer means in said second circuit, resistor means in series with said potentiometer across the power supply, a small capacity condenser in 12 said second circuit and releasably connected with said storage first condenser through said switch means, and a null indicating meter means connected in series with said memory second condenser across the terminals of the said potentiometer to provide a null reading upon the balancing of the charge on said memory,l second capacitor by the adjustment of said potentiometer that then indicates the duration oi' the time interval during which said nrst thyratron applies charge to said storage nrst condenser, and means for substantially removing charge from said storage tlrst condenser.

EARL G. NEWSOM.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS OTHER REFERENCES Publication "Academi Des Sciences," July 9. 1934, pages 123-126. 

